Plasticizers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, toys, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticizers include polyvinyl butyral, acrylic polymers, poly(vinylidene chloride), nylon, polyolefins, polyurethanes, and certain fluoroplastics. Plasticizers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers). A listing of the major plasticizers and their compatibilities with different polymer systems is provided in “Plasticizers,” A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp. 157-175.
Plasticizers can be characterized on the basis of their chemical structure. The most important chemical class of plasticizers is (ortho-)phthalic acid esters, which accounted for about 85% worldwide of PVC plasticizer usage in 2002. However, in the recent past there as been an effort to decrease the use of phthalate esters as plasticizers in PVC, particularly in end uses where the product contacts food, such as bottle cap liners and sealants, medical and food films, or for medical examination gloves, blood bags, and IV delivery systems, flexible tubing, or for toys, and the like. For these and most other uses of plasticized polymer systems, however, a widely accepted substitute for phthalate esters has heretofore not materialized.
One such suggested substitute for phthalates are esters based on cyclohexanoic acid. In the late 1990's and early 2000's, various compositions based on cyclohexanoate, cyclohexanedioates, and cyclohexanepolyoate esters were said to be useful for a range of goods from semi-rigid to highly flexible materials. See, for instance, WO 99/32427, WO 2004/046078, WO 2003/029339, WO 2004/046078, U.S. Application No. 2006-0247461, and U.S. Pat. No. 7,297,738.
However, one of the problems with plasticizers based on esters of cyclohexanoic acid is processability, particularly the fusion characteristics. When a plasticized product is produced, such as a PVC product, the product should reach a temperature at some point during fabrication at which the polymer crystallites are melted. This is called the fusion temperature. In the case of PVC, depending upon the plasticizer, this temperature generally ranges from 160 to 180° C. Plasticizers which are better solvents for a given polymer will fuse at lower temperatures than those that are poorer solvents. Since many plasticized polymer products, such as flexible PVC products, are produced through continuous processes, those faster or stronger solvating plasticizers will arrive at this fusion temperature faster; hence the development of the descriptor “fast fusing” or “faster fusing”. These same plasticizers are also known as strong solvating plasticizers. For most applications, the plasticizer reference standard is di-2-ethylhexyl phthalate (DEHP) as this plasticizer has been the most widely used plasticizer world wide since it was commercialized in the late 1930's. Plasticizers which fuse at lower temperatures than that required for DEHP, at the same concentration in a given polymer system, are considered fast fusing plasticizers. Likewise, plasticizers that fuse at higher temperatures than that required for DEHP, at the same concentration in a given polymer system, are considered “slow fusing” plasticizers.
It has been proven to be particularly difficult to identify, develop and commercialize a widely accepted fast fusing plasticizer substitute for phthalate esters. Fast fusing plasticizers are defined in more detail further in this document.
Fast fusing plasticizers are valued in the production of many flexible articles, particularly flexible PVC articles. See, for instance, U.S. Pat. No. 7,297,728. Fast fusing plasticizers based on non-phthalates are also known. For instance, the present inventors have recently described, along with others, fast fusing plasticizers based on cyclohexanoic acid esters of C4-C7 secondary alcohols (see copending application PCT/US2008/080891, filed Oct. 23, 2008), plasticizers based on cyclohexanoic acid esters of C7-C12 secondary alcohols (see copending application PCT/US2008/080893, filed Oct. 23, 2008), and also coplasticizer systems based on cyclohexanoic acid esters and non-phthalate fast fusing plasticizers. See also U.S. Pat. No. 7,323,588.
Other suggested substitutes for phthalates as plasticisers include esters based on benzoic acid (see, for instance, U.S. Pat. No. 6,740,254 or WO 2006/077131) and polyketones, such as described in U.S. Pat. No. 6,777,514; and also in WO 2008/121847. Epoxidized soybean oil, which has much longer alkyl groups (C16 to C18) has been tried as a plasticizer, but is generally used as a PVC stabilizer. Stabilizers are used in much lower concentrations than plasticizers.
Typically, the best that has been achieved with substitution of the phthalate ester with an alternative material is a flexible PVC article having either reduced performance or poorer processability. Thus, heretofore efforts to make phthalate-free plasticizer systems for PVC have not proven to be entirely satisfactory, and this is still an area of intense research.
Triglycerides produced from branched C6 to C9 acids have been studied in the past, but primarily in other technical fields and the studies have been rather limited in scope. W. Keil, in “Zur Kenntnis der Fette aus Fettsäuren mit ungerader Kohlenstoffatomzahl”, Hoppe-Seyler's Zeitschrift für Physiologische Chemie, vol. 282, 1947, pages 137-142 studied the metabolites of a triglyceride of 2-propyl pentanoic acid and of 3-propyl hexanoic acid when fed to dogs, by urine analysis. A. Aydin et al., in “The synthesis of mono- and triglycerides of branched fatty acids and physical properties of the synthesized glycerides.”, Chimica Acta Turcica, vol. 5, 1977, pages 93-101, determined physical properties of 2-ethyl hexanoic acid and of 2-propyl hexanoic acid, and predicted usefulness of such triglycerides in numerous future applications in various fields of industry, more specifically in textile industry as softening material in sanforization, i.e. a mechanical shrinking process for fabrics before these are manufactured into articles such as clothing, in cosmetic formulations and in the food industry.
Polyol esters of branched C6 to C9 acids are known as lubricants and lubricant components. A. D. Godwin et al. in U.S. Pat. No. 6,307,093 disclose in col. 12 as useful in this field the esters of branched C9 acids with pentaerythritol, di(pentaerythritol), tri(pentaerythritol); trimethylolethane, trimethylolpropane, trimethylolbutane, and dimers and trimers thereof, and neopentylglycol.
Plasticizers based on triglycerides have been tried in the past, based on natural triglycerides from various vegetable oils. The alkyl groups on these natural triglycerides are linear, and the products can be incompatible when the alkyl chain is too long.
“Structural Expressions of Long-Chain Esters on Their Plasticizing Behavior in Poly(vinyl Chloride)”, H. K. Shobha and K. Kishore, Macromolecules 1992, 25, 6765-6769, reported the influence of branching and molecular weight in long-chain esters in PVC. Triglycerides (TGE's) having linear alkyl groups were studied.
“A Method for Determining compatibility Parameters of Plasticizers for Use in PVC Through Use of Torsional Modulus”, G. R. Riser and W. E. Palm, Polymer Engineering and Science, April 1967, 110-114, also investigate the use of triglycerides and their plasticizing behavior with PVC, including tri-iso-valerin (3-methyl butanoate) triglyceride. It was reported that “these materials have volatilities that are much too high for good long-time permanence”.
Nagai et al. in U.S. Pat. No. 5,248,531, teaches the use of articles comprising vinyl chloride-type resins (among others) using triglyceride compounds as a hemolysis depressant, and also comprising 10 to 45 wt % of plasticizers selected from trialkyl trimellitates, di-normal alkyl phthalates, and tetraalkyl pyromellitates. The alkyl chains of the acid moiety R1-R3 in the structure below, formula (I), are independently an aliphatic hydrocarbon group of 1 to 20 carbon atoms and in embodiments at least one of the alkyl chains is branched. One specific triglyceride disclosed is glyceryl tri-2-ethylhexanoate, having the following formula (I).

Zhou et al. discloses, in U.S. Pat. Nos. 6,652,774; 6,740,254; and 6,811,722; phthalate-free plasticizers comprising a mixture of different triesters of glycerin, formed by a process of esterifying glycerin with a mixture comprising a mixture of alkyl acids and aryl acids. A triglyceride ester produced from a 50/50 mixture of 2-ethyl-hexanoic acid and benzoic acid is exemplified. It was found to be compatible with PVC resin, while glyceryl tribenzoate and glyceryl tri(2-ethyl)hexanoate are stated in paragraph [0020] to be known as being incompatible in such resin.
Nielsen et al., in U.S. Pat. No. 6,734,241, teach a composition comprising a thermoplastic polymer as in formula (I) above, wherein at least one of the R groups is a short alkyl group having from 1-5 carbon atoms and at least one of the R groups is a saturated branched alkyl group having from 9 to 19 carbon atoms and also having a hydrophilic group.
U.S. Pat. No. 6,740,254 mentions plasticizer esters based on C4 and benzoic acids.
However, the prior art has not recognized the advantages of using esters, including triglycerides, based on polyols together with particularly selected branched alkyl acids as plasticizers, and in combination with aryl acids as fast fusing plasticizers. The latter may be used with other slower fusing plasticizers in plastics systems. It was stated already hereinbefore that the TGE of 2-ethyl hexanoic (2EH) acid has a problem of compatibility with PVC. A particular problem with esters from alkyl acids having an ethyl branch on the second carbon is a toxicity concern. An ester, when introduced into a living organism such as a human or an animal, may become at least partly hydrolysed. The ester hydrolysis liberates the acid. When the ester comprises high amounts of the 2-ethyl hexyl moiety on the acid alkyl group, 2-ethyl hexanoic acid is liberated in significant amounts in the living organism, which is undesirable because of the toxicity concern associated with this specific acid (Manninen et al, 1989, Archives of Toxicology 63(2), pages 160-1). This effect may be attributed to the specificity of the ethyl branch on the second carbon, and the concern may therefore also relate to other acids having that branch in that location.
Among the problems presented by the aforementioned triglycerides is they cannot be made conveniently and thus generally are quite expensive and/or are specialty chemicals not suitable as replacements for phthalates from an economic standpoint and/or are not as compatible with the range of polymer systems that phthalates are compatible with, and thus are not viable general purpose replacements for phthalates from a physical property standpoint.
For instance, some synthesis methods involve at least two separate steps, such as where the glycerol is first partially esterified with the C10 to C20 branched chain acyl group and then reacted with acetic acid or acetic anhydride.
Other syntheses involving mixed acid feeds will require addition of a hydrocarbon solvent for azeotropic distillation of the water to drive the esterification reaction to completion (as measured by the hydroxyl number of the ester, which is a measure of the amount of unreacted OH groups), due to the spread in boiling points between the mixed acids. In addition, the use of mixed acid feedstock such as cited in Zhou et al. and in Nielsen et al. can increase the process complexity when recycling unreacted acids.
Triglycerides based on acids derived from natural products will be limited to naturally occurring linear alkyl groups with even carbon numbers, which offer very little flexibility in designing an appropriate plasticizer system for a given polymer system.
Thus what is needed is a method of making a general purpose non-phthalate plasticizer having high throughput and providing a plasticizer having suitable melting or pour point, increased compatibility, good general purpose performance and low temperature properties.
The present inventors have surprisingly discovered that triglyceride and or glycol esters, produced by esterification of glycerol, ethylene glycol or propylene glycol with acids derived from the hydroformylation of olefins and subsequent oxidation of the oxygenate to a branched C6 to C9 acid, provide for polyol esters having appropriately branched alkyl groups for providing compatibility with a wide variety of resins and which are obtainable with a high throughput. Esterification of glycerol using an acid mixture with a narrow carbon number range eliminates many of the aforementioned problems, and enables high yield of the glycerol triesters to be obtained, having low residual hydroxyl numbers.